docs: def _readbytes(self, length, start):
          """Read bytes and return them. Note that length is in bits."""
          assert length % 8 == 0
          assert start + length <= self.len
          if not (start + self._offset) % 8:
              return bytes(self._datastore.getbyteslice((start + self._offset) // 8,
                                                        (start + self._offset + length) // 8))
          return self._slice(start, start + length).tobytes()